Exhaust aftertreatment devices for reducing emissions from internal combustion engines are well known. It is known in the diesel engine art to provide in series a plurality of exhaust aftertreatment devices, referred to herein for simplicity as AD1 and AD2. Especially in treatment of diesel engine exhaust, such devices are designed to collect or trap undesirable exhaust constituents such as particulates or NOx, becoming full over time. They may also become contaminated by exhaust constituents which inactivate the aftertreatment device chemically, such as sulfur, or physically, such as ash, which can cause clogging or other dysfunction from prolonged exposure to the exhaust stream. Thus, it is important to be able to clean, or “regenerate,” inline exhaust amelioration devices as needed, while the engine is running.
It is further known in the prior art to provide a catalytic hydrocarbon reformer for generating hydrogen-rich reformate which is added to the engine exhaust stream upstream of the aftertreatment devices. The hydrogen attacks and removes deposits in the devices. In a typical cleaning duty cycle for an 8-cylinder light duty diesel vehicle, reformate is introduced into the exhaust stream for approximately 10 seconds, followed by approximately 70 seconds of little or no reformate. Typically, about 20 grams per second of reformate is needed for adequate regeneration.
In a prior art arrangement, the reformer takes in hydrocarbon fuel and fresh air to produce the reformate. To inject this reformate into the exhaust stream ahead of the aftertreatment devices requires that the pressure of the reformate be higher than the exhaust backpressure, Pengine, at all speeds and loads, so that the reformate will flow into the exhaust stream. The apparatus must include a pump to raise the pressure of the reformate output stream to a pressure of approximately 80–100 kPa above ambient pressure, Pambient, which is the nominal inlet air pressure for the engine and the reformer, to overcome the exhaust backpressure. If the pump is 80% efficient, for example, an electric motor of about 1.5 kW input is required to run the pump. This size electric motor is large, expensive, and not practically powered by conventional 12–14 volt electrical systems provided in typical vehicles.
What is needed in the art is an improved method and apparatus for providing reformate into an engine exhaust stream which reduces the required size of the pump and pump motor.
It is a principal object of the present invention to reduce the pressure head against which a reformate pump must operate, thereby reducing the required size of the pump and pump motor.